Answer: C.225
Step-by-step explanation: for this case we need to calculate the value of x, to complete this we have 2 equations,
you can stat with simple equation X+y=525, if you clear x we get the following equation.
x = 525 - y
after that we are going to replace x in the major equation.
3x + 7y = 2775
After replace x we get the following equation
3 * (525 - y) + 7y = 2775
after that, the 3 is going to multiply each value into the parenthesis
3 * 525 - 3 * + 7y = 2775
after resolve the product we get the following equation
1575 - 3y + 7y = 2775
you need resolve the equation with values which has the y
getting the following result
1575 + 4y = 2775
after that you pass the 1575 to the other side with the sign of subtraction
4y = 2775 - 1575
4y = 1200
after that, the 4 which is multiplying pass to the other side dividing
then
y = 1200/4
y = 300
finally, you will replace the value of y in the equation where we clear x, this equation.
x = 525 - y
if you replace y
x = 525 - 300
x = 225
then value of the x is 225